I would recommend that your boyfriend gets a copy of his credit report. I work for a bank, and I hear customers say "I am sure that my credit rating is poor...." just because they have slipped overdrawn once or twice and incurred one of those beastly charges that I am always refunding; when in fact their rating is perfectly good.
Also, if your boyfriend has been declined for credit, find out WHY (lenders are legally obliged to tell you this). It may be that he has failed the lender's scoring criteria which is not necessarily a bad thing at the time. If he has been declined owing to an adverse credit search on his name, then yes he does have bad credit.
Ethel makes an excellent point (as always - she is pretty good on this site!) with the financial association. I have seen a customer apply for a loan at work which has been declined on the grounds of her joint account conduct which is managed by her boyfriend, yet her sole account is managed perfectly well. I overturned the decision and agreed the loan for her as I could see how she managed her financial affairs.
I would recommentd you get a copy of his credit report. If there is information on the report which is causing the application to be declined which is factually incorrect (financial institutions DO make mistakes - they are human!), then you can get it removed. The below link should assist you with this:
http://www.experian.co.uk/
Good luck, and all the very best.
Kind regards,
Scott